Roof Rats and Norway Rats in California: Why the Species Matters

What does UC IPM say about the two rats?

UC IPM’s Pest Notes on rats, revised in June 2024, state that roof rats and Norway rats are present in almost all California cities. The notes say knowing which species is present determines how the rat should be controlled.

That is a statewide statement, and it is the basis of this guide. It does not give a count for any city, and it does not say which species is more common on a particular street. The guide does not add a figure the notes do not give.

The point that matters for a homeowner is the second statement. A control chosen without knowing the species can miss the animal that is actually using the house, and the identification is therefore the first technical step on any rat visit.

How do the two rats live?

A roof rat favours height and uses the roof line, the eaves, trees and overhead wires. A Norway rat favours the ground and uses burrows, lower walls and the space under a slab, deck or shed.

A roof rat is comfortable high in a building. It runs along the top of a fence, climbs a tree that touches the roof and enters the attic at a vent or a soffit gap. Its droppings are often found high, and its nests are built in attic and wall spaces.

A Norway rat lives at ground level. It digs or uses burrows at a fence line, under a shed or a deck, and it moves along the lower walls and the drains. Its droppings and runs are more often at ground level, and its entry is more often a gap at the foundation or a burrow at the base of a wall.

How do the signs differ on a property?

The signs differ in height and place. Attic and eave signs point toward the roof rat, and burrows and lower-wall runs point toward the Norway rat. Where the signs are mixed, the identification is recorded as open.

A roof rat leaves droppings in the attic and along roof boards, and it leaves grease marks along joists and beams at the height of its body. A Norway rat leaves burrows with fresh soil at the mouth, grease marks along the base of walls and droppings in the garage and the crawl space.

A single sign is weak evidence. A cluster of droppings in one place, a run that stays at one height and a burrow with a worn path all carry more weight, and the identification weighs them together.

How is the identification confirmed?

Where the signs do not decide the question, a specimen or a short trapping record helps. The trapping record shows which animals use which route, and it is the evidence the identification needs when the signs are mixed.

A specimen that is found dead is kept cool and sealed. It is examined for the features the UC IPM notes describe, and the report records whether the examination was conclusive. A damaged specimen is not forced into a conclusion.

A trapping record places traps on the runs the evidence points to, for a set number of nights. The catch and the position are recorded each morning. Several nights of clear catches usually settle the question for a house.

Why does the species change the sealing?

A roof rat uses the eaves and the roof line, so the sealing is planned there. A Norway rat uses the foundation and the ground, so the sealing is planned at the rim joist, the weep holes and the garage joint.

A sealing plan written for ground entries will miss the soffit and the gable vent that a roof rat uses. A plan written only for the roof line will miss the rim joist and the gap under the garage door that a Norway rat uses. The species tells the technician which of those to do first.

Where both species are possible, the sealing covers both, and the devices are placed on both routes until the identification is settled.

What should a Fremont homeowner do?

Keep the dated notes of the signs, photograph the droppings and the marks with a scale, keep food and pet food in sealed containers and ask the technician to record the species and the evidence.

The homeowner’s notes are the best starting point for the identification. A dated photo of a dropping beside a coin, a note of where it was found and a note of the height all help the technician place the evidence.

A homeowner should not set a trap or a bait before the identification without advice, because a device on the wrong route can be wasted and can put a product where a pet can reach it.

How do the two rats move through a Fremont neighbourhood?

Each species moves along the lines a building offers. A roof rat follows fences, trees and wires across the top of a lot, and a Norway rat follows the ground, drains and the edges of buildings. A neighbourhood is therefore a network of routes that both can use.

A fence line between two lots is a route for either species. A roof rat walks along its top, and a Norway rat travels along its base or through a gap. A tree that overhangs two houses is a route for the roof rat, and a drain that runs between two lots is a route for the Norway rat.

The routes across a neighbourhood matter for the sealing. Closing one house’s entry can push the animal to a neighbour’s, which is why the identification and the route are discussed with the neighbour where the route crosses the line.

What does a rat visit look like from start to finish?

A rat visit starts with the evidence, moves to the identification, places the devices on the confirmed routes and seals the entries. A follow-up checks the result and sets the next step. The order is the same whether the house is old or new.

The visit begins by reading the signs and asking the homeowner about the dates. The walk then follows each sign to the route it marks. The identification is recorded before any device is placed, and the devices are placed on the routes the identification supports.

The sealing comes after the devices are in place, and the entries are closed in the order the routes require. The follow-up checks each closure and each device, and the next step is set from what is found.

Why is a monitoring period sometimes the right answer?

Where the identification is open, or where the routes are not yet clear, a monitoring period records the animals that use the house without assuming the result. The period is short and it is set in the plan.

A monitoring period places traps or monitors on the likely routes and records the catch for a set number of nights. The record shows which animals are using which routes, and it is the evidence the identification needs.

A homeowner who is asked to accept a monitoring period should expect a date for the review of the record and a clear statement of what the review will decide.

Garages, fences and the ground level

At ground level the Norway rat finds its way in through gaps at the garage door, the joint between the garage and the house, and openings in the fence and the foundation. The walk checks each of those points and records the species evidence beside it.

The garage is the most common ground-level entry in a Fremont house. A worn door seal, a gap under the threshold and a joint where the garage wall meets the house are all used by rats. The walk checks each one from inside the garage with a light, and it records the gaps by position.

The fence line and the garden edge are the Norway rat’s route to the foundation. A burrow at the base of a post, a worn run along the boards and a gap where the fence meets the house are the signs. The report records each, and the sealing list shows which can be closed at once and which need the neighbour’s agreement.

Under a deck or a shed, the Norway rat finds cover and a route to the foundation. The walk checks the space under the deck and the shed, and it records any burrow or nest. The space is cleared where it is safe, and the openings are closed with metal.

The roof line and the tree contact

At the roof line the roof rat finds its way in through the soffit, the gable vent and the points where a branch or a wire touches the house. The walk checks each point from the ground and, where it is safe, from a stable ladder.

The soffit is the most common roof-line entry. A torn screen on a soffit vent, or a gap at a soffit joint, gives the roof rat a way into the attic space. The walk checks each vent and each joint, and it records the position of each gap for sealing with metal.

Trees are the roof rat’s highway. A branch that touches the roof gives the animal a route from the tree to the eave, and the report recommends that a qualified tree service cut the branch back. The contact is recorded on the plan, and the sealing is planned for the eave beneath it.

The gable vent is often missed. A vent with a bent or missing screen gives access to the roof space, and the walk checks its screen and its frame, recording the repair needed.

What a homeowner can check between visits

Between visits the homeowner can walk the outside of the house once a month, with a torch, looking for new droppings, fresh grease marks, a new burrow or a sound in the roof. A short dated note of each check is the most useful record for the next visit.

A short walk along the fence, the garage wall and the eaves, once a month, takes less than fifteen minutes. The homeowner looks for droppings in the corners, grease marks on the boards and any new gap in a screen or a seal. The note records the date, the place and what was seen, and the note goes into the house file.

A homeowner who finds a sign between visits should not disturb it. A burrow left alone can be read by the technician, and a dropping left in place can be sampled. Disturbing the sign makes the identification harder, and the report depends on the sign being intact.